1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing equipment, and more specifically, to an apparatus and method for depositing a film at a rate which is independent of wafer characteristics.
2. Discussion of Related Art
During the manufacture of semiconductor devices, thin films are often formed on a semiconductor wafer surface. A typical processing apparatus 110 for depositing a thin film on a semiconductor wafer is illustrated in FIG. 1A. Processing apparatus 110 will typically include a susceptor 102 located within a chamber 119 on which a wafer or substrate 104 to be processed is placed. The top 106 and bottom 108 of reaction chamber 119 are generally formed of quartz to allow light of the visible and IR frequency from lamps 126 to pass into chamber 119 and heat susceptor 102 and wafer 104. A non contact temperature measurement device 114, such as a pyrometer, is provided to measure the temperature of susceptor 102. The temperature of wafer 104 is assumed to be at the same temperature of susceptor 102.
Pyrometer 114 senses the amount of radiation emitted from susceptor 102 and provides a related voltage signal to computer 118, from which computer 118 calculates the temperature of susceptor 102. Computer 118 then signals power controller 116 to either increase or decrease the power to lamps 126 in order to increase or decrease the temperature of susceptor 104.
A problem with the above referenced deposition apparatus and method of operation is that the deposition rate for a given process (i.e., constant pressure, power, flow rate, etc.) is dependent upon the specifics of a wafer characteristic. The above referenced deposition apparatus represents an inherently non-equilibrium situation in that the wafer and susceptor are at an elevated temperature with respect to the chamber walls. The extent of the non-equilibrium situation depends on the emissivity of the wafer surface since this determines the heat input versus heat loss from the wafer. Since the growth rate of polysilicon deposition depends strongly on the surface temperature, varying heat losses (in the form of emissivity differences) leads to variations in film thicknesses for different wafers with varying surface characteristics. For example, the deposition rate of a polysilicon film for a given process will depend upon the initial thickness of an oxide grown on the wafer. That is, the polysilicon deposition rate on a wafer having a 1000 .ANG. thick oxide formed thereon will be different than a wafer having a 100 .ANG. thick oxide formed thereon. When the deposition rate of a process varies with specific wafer characteristics, wafer to wafer repeatability suffers and the process is not manufacturable.
Other factors that affect the surface characteristics (and hence the emissivity) are the density of patterns in a thin film on the wafer, film roughness and multiple layers of stacked films. For example, a wafer having a dense pattern of electrodes or vias across the surface will have a different deposition rate than a wafer with a less dense pattern. Similarly, wafers having a film with a rough surface will have a deposition rate different than a wafer having a film with a smooth surface. Like different oxide thickness, pattern densities and surface roughness affect the emissivity of the wafer which in turn causes a variation in the surface temperature of the wafer which in turn leads to variations in the deposition rate.
While the wafer is in contact with the above described susceptor, the above surface temperature variation for wafers with different surface characteristics is minimized. However, when the wafer is held at a distance from the susceptor (such as in a "pin" susceptor configuration), the surface temperature variation is exacerbated. This effect can be seen in FIG. 1b where the deposition rate of a given process varies with the surface characteristic of the wafer, in this case an initial oxide thickness variation on the silicon wafer. While the susceptor temperature and susceptor emissivity can be controlled in such a system, the deposition system has no control over the condition of the incoming wafer. Hence, a fixed power distribution that preferentially heats the susceptor with the bottom lamps and the wafer with the top lamps can lead to the effects exhibited in FIG. 1b.
Thus, what is desired is a method and apparatus for depositing a film on wafer wherein the deposition rate is independent of specific wafer characteristics or profiles.